1. Field of the Invention
The present invention relates to a method of producing a mold having an uneven structure, a mold for an optical element, and an optical element. In particular, an optical element molded using a mold for an optical element according to the present invention has a function of suppressing the amount of interfacial reflection of light from incident and exit surfaces. For example, the optical element is suitable for imaging apparatuses such as cameras and digital camcorders, and projection apparatuses such as liquid crystal projectors and panels and optical scanning devices of electrophotographic apparatuses.
2. Description of the Related Art
In general, in an optical element for which suppression of the amount of surface reflection is required, a single layer or a plurality of stacked layers of optical film with different refractive indexes having a thickness of several ten nanometers or several hundred nanometers is formed on a surface thereof, whereby desired reflection characteristic are obtained. The optical film is formed by use of a vacuum film-forming process such as vapor deposition or sputtering or a wet film-forming process such as dip coating or spin coating. In any of those film-forming processes, since the film formation needs to be performed after working of an optical element substrate, the production is difficult, thus resulting in a limitation in cost reduction.
On the other hand, it is known that the amount of interfacial reflection can be suppressed by forming on a surface of an optical element a fine shape at a pitch equal to or less than the design wavelength, without use of an optical film. If this principle is utilized to form a fine shape in a mold and an optical element can be produced concurrently with the molding of a substrate, the production cost can be reduced accordingly.
As a technique of forming a fine shape called SWS (SubWave-length Structure), there has been hitherto widely used the semiconductor processing method which advantageously enables a precisely designed SWS to be formed. However, there has been the problem that in the case of forming a fine shape on a curved surface with a large area, this technique has many restrictions and low-cost (or simple) production is extremely difficult.
On the other hand, as a simple technique of producing an SWS, there has been proposed a technique of producing an SWS by utilizing fine particles (see Japanese Patent Application Laid-Open Nos. 2000-071290 and 2001-074919, for example). In Japanese Patent Application Laid-Open No. 2000-071290, in the case of utilizing fine particles, an SWS can be formed entirely over a large area. However, there has been the problem that since fine particles need to be arranged continuously evenly to constitute an SWS, it is difficult to control a volume ratio between a substrate and an atmosphere and an aspect ratio which determine the reflection characteristics and an ideal reflection-preventing effect is difficult to obtain.
On the other hand, as a technique of forming an SWS in a large area and arbitrarily controlling the aspect ratio thereof, an anodization method is known. In an acidic electrolyte solution, a metal such as aluminum is used as an anode and oxidized by causing a current to flow therethrough to thereby form fine pores therein. By utilizing this, there have been developed a technique of arranging pores regularly, a technique of filling pores with a different material and the like (see, for example, Japanese Patent Application Laid-Open Nos. H02-254192 and H1-121292).